Cooling structure of cylinder block and swash plate type liquid-pressure apparatus including same

ABSTRACT

The present invention provides a cooling structure of a cylinder block, the cooling structure being capable of improving a cooling performance of sliding surfaces. A cylinder block includes a plurality of cylinders, and pistons are respectively inserted into the cylinders through openings. Each of the pistons performs reciprocating sliding on a sliding surface which defines the cylinder. A plurality of cooling depressions are formed on an outer peripheral surface of the cylinder block. Each of the cooling depressions extends from a front end surface of the cylinder block on a dividing wall located between the two adjacent cylinders and is formed by reducing the thickness of the dividing wall so as to reduce a thickness of a portion between the sliding surface and the outer peripheral surface.

TECHNICAL FIELD

The present invention relates to a cooling structure of a cylinderblock, such as a cylinder block of a swash plate type liquid-pressureapparatus, configured such that: a plurality of cylinders are formed onthe cylinder block; pistons can be respectively inserted throughopenings of the cylinders, the openings being formed on a pistoninsertion end surface of the cylinder block; and the inserted pistonsperform reciprocating sliding in the cylinders when the cylinder blockis rotated.

BACKGROUND ART

Various oil-pressure motors and oil-pressure pumps are used inindustrial machinery, such as construction machinery. As one example ofthe oil-pressure motors and oil-pressure pumps, a swash plate typeoil-pressure motor/pump (hereinafter may be referred to as a “swashplate type oil-pressure apparatus”) as in PTL 1 is known. The swashplate type oil-pressure apparatus of PTL 1 includes a rotating shaft,and a cylinder block is integrally attached to the rotating shaft.Cylinders are formed on an end surface of the cylinder block so as to bearranged at regular intervals in a circumferential direction, andpistons are respectively inserted into the cylinders. Shoes arerespectively attached to end portions of the pistons, the end portionsprojecting from the cylinders. The shoes are arranged on a supportingsurface of a swash plate provided to be inclined.

In the swash plate type oil-pressure apparatus configured as above, thecylinder block is rotated by the reciprocating movements of the pistonsin the cylinders. By supplying high-pressure operating oil to thecylinders, the pistons perform the reciprocating movements, and thisrotates the cylinder block. Thus, the rotating shaft formed integrallywith the cylinder block is rotated by the rotation of the cylinderblock. To be specific, the swash plate type oil-pressure apparatusserves as the oil-pressure motor. In addition, in the swash plate typeoil-pressure apparatus, by rotating the cylinder block, the pistonsperform the reciprocating movements in the cylinders. By rotating thecylinder block by the rotating shaft, the swash plate type oil-pressureapparatus can suction the low-pressure operating oil and eject thehigh-pressure operating oil. To be specific, the swash plate typeoil-pressure apparatus can also serve as the oil-pressure pump.

CITATION LIST Patent Literature

PTL 1: Japanese Laid-Open Patent Application Publication No. 2010-174690

SUMMARY OF INVENTION Technical Problem

The swash plate type oil-pressure apparatus configured as in PTL 1 hasbeen mainly used in low-speed rotation and medium-speed rotation.However, it is desired that in order to respond to the increase in therotation of a driving device of the construction machinery or theindustrial machinery, the swash plate type oil-pressure apparatus isconfigured to be able to be used in high-speed rotation. However, if thecylinder block of the swash plate type oil-pressure apparatus is rotatedat high speed, the influence of centrifugal force acting on the pistonsand the shoes increases and becomes unignorable, unlike a case where thecylinder block of the swash plate type oil-pressure apparatus is rotatedat low speed.

For example, when the pistons perform the reciprocating movements in thecylinders, heat is generated by the sliding of the pistons on slidingsurfaces of the cylinder block. The amount of heat generated on thesliding surfaces depends on the contact pressure between the pistons andthe cylinder block. In a conventional low-speed rotation type in whichthe centrifugal force is extremely small, the contact pressurecorresponds to the pressure of the operating oil supplied or ejected.Therefore, the heat generated on the sliding surfaces is comparativelysmall in amount. On this account, the sliding surfaces can be adequatelycooled only by the operating oil leaking from clearances each formedbetween the sliding surface and the piston for allowing the operatingoil to escape.

However, when the cylinder block is rotated at high speed, the influenceof the centrifugal force on the contact pressure becomes larger than theinfluence of the oil pressure on the contact pressure. As the rotatingspeed increases, the contact pressure increases, and the amount of heatgenerated on the sliding surfaces also increases. With this, thetemperatures of the sliding surfaces increase, and the cooling by theoperating oil leaking from the clearances becomes especially difficult.On this account, the temperature increase in the vicinity of theopenings of the cylinders become significant. In addition, since thepistons are pressed outward by the increase in the centrifugal force,the width of each of radially outer clearances of the cylinder blockbecomes smaller than the width of each of radially inner clearances ofthe cylinder block. In this case, the flow of the operating oil at thenarrow radially outer clearances becomes nonsmooth, and the operatingoil is heated at the narrow radially outer clearances. If the operatingoil is continuously heated, and the temperature thereof exceeds atransition temperature of the operating oil, a lubrication performanceof the operating oil decreases. With this, the amount of heat generatedon the sliding surfaces further increases, and the cylinders and thepistons may be burned out. By increasing the widths of the clearances,the decrease in the lubrication performance of the operating oil and theburnout can be prevented. However, if the widths of the clearances areincreased, the leakage amount of operating oil significantly increases.Therefore, the performance as the pump or motor deteriorates, and theincrease in the pressure of the oil-pressure apparatus is limited.

Here, an object of the present invention is to provide a coolingstructure of a cylinder block capable of improving a cooling performancefor sliding surfaces.

Solution to Problem

A cooling structure of a cylinder block according to the presentinvention is configured such that: a plurality of cylinders eachincluding an opening on a piston insertion end surface of the cylinderblock are formed on the cylinder block; and when the cylinder block isrotated, pistons respectively inserted in the cylinders performreciprocating sliding, the cooling structure including: a plurality ofcooling depressions formed on an outer peripheral surface of thecylinder block, wherein each of the cooling depressions extends from thepiston insertion end surface on a dividing wall located between the twoadjacent cylinders and is formed by reducing a thickness of the dividingwall so as to reduce a thickness of a portion between the outerperipheral surface of the cylinder block and a sliding surface on whichthe piston slides.

According to the present invention, the thickness of the portion betweenthe sliding surface and the outer peripheral surface becomes small.Since the temperature of the sliding surface in the vicinity of theouter periphery which generates heat by the centrifugal force generatedby the high-speed rotation is higher than the temperature of drain oilin a case on the periphery of the sliding surface. Therefore, the heatgenerated on the sliding surfaces can be quickly transferred to theouter peripheral surface and thus can be released from the outerperipheral surface. With this, the cooling performance of the slidingsurfaces can be improved, and the temperature increase of the slidingsurfaces can be suppressed. In addition, since the cooling depressionsextend from the piston insertion end surface on which the openings ofthe cylinders are located, the increase in the surface temperature of aportion of the sliding surface can be especially suppressed, the portionincreasing in temperature most significantly and being located in thevicinity of the piston insertion end surface. Therefore, the occurrenceof the burnout of the sliding surface can be suppressed.

For example, in a case where the present invention is used in aliquid-pressure apparatus, such as an oil-pressure pump or anoil-pressure motor, a clearance is formed between the sliding surfaceand the outer peripheral surface of the piston, and the operating oilleaking from the clearance is utilized as lubricating oil. Bysuppressing the temperature increase of the sliding surface, theincrease in the temperature of the lubricating oil can be suppressed,and the transition of the lubricating oil can be prevented. With this,since the decrease in the lubrication performance of the lubricating oilcan be prevented, smooth movements of the pistons can be maintained, andthe amount of heat generated on the sliding surfaces can be reduced.

In the above invention, it is preferable that: each of the pistonsperform the reciprocating sliding between a top dead center and a bottomdead center in the cylinder; and each of the cooling depressions beformed so as to extend from the piston insertion end surface in parallelwith the cylinder and be formed such that a tip end of the coolingdepression is located on the piston insertion end surface side of avicinity of an end surface of the piston located at the bottom deadcenter, the end surface being located in the cylinder.

According to the above configuration, while maintaining the stiffness ofa portion where the pressure becomes high when the piston is located atthe bottom dead center, the cooling performance of a region where thesurface temperature becomes high can be improved. With this, damages bythe burnout of the cylinder and the piston can be prevented withoutdecreasing a service limit pressure of the operating liquid supplied tothe cylinder block.

In the above invention, it is preferable that each of the coolingdepressions be formed so as to satisfy 0.02D≦tmin≦0.3D, where tmindenotes a minimum thickness of the portion between the outer peripheralsurface of the cylinder block and the sliding surface, and D denotes aninner diameter of the cylinder.

According to the above configuration, the stiffness of a region of thesliding surface can be secured while improving the cooling effect, theregion being located on the outer peripheral surface side. With this,the burnout of the cylinder block and the damages on the opening sidecan be prevented.

A swash plate type liquid-pressure apparatus of the present invention isconfigured to be connected to a low-pressure passage through which alow-pressure operating liquid flows and a high-pressure passage throughwhich a high-pressure operating oil flows and further configured suchthat: the cylinder block is rotated by supplying the operating liquidthrough the high-pressure passage to the cylinders and discharging theoperating liquid from the cylinders to the low-pressure passage; or byrotating the cylinder block, the operating liquid is suctioned throughthe low-pressure passage to the cylinders, and the operating liquid isthen compressed and ejected to the high-pressure passage, and the swashplate type liquid-pressure apparatus includes any one of the abovecooling structures.

In the swash plate type liquid-pressure apparatus, the clearance isformed between the sliding surface and the outer peripheral surface ofthe piston, and the operating oil leaking from the clearance is utilizedas the lubricating oil. According to the above configuration, bysuppressing the temperature increase of the sliding surface, theincrease in the temperature of the lubricating oil leaking from theclearance can be suppressed, and the transition of the lubricating oilcan be prevented. With this, the decrease in the lubrication performanceof the lubricating oil can be prevented, the smooth movements of thepistons can be maintained, and the amount of heat generated on thesliding surfaces can be reduced.

In the above invention, it is preferable that the swash plate typeliquid-pressure apparatus further include a casing configured toaccommodate the cylinder block, wherein the casing is connected to thelow-pressure passage through a communication passage, and low-pressureoperating oil in the low-pressure passage is introduced to the casing,

According to the above configuration, since the outer peripheral surfaceof the cylinder block can be subjected to low-pressure, low-temperatureoperating liquid introduced to the casing, the outer peripheral surfacecan be cooled by the operating liquid. With this, a larger amount ofheat can be released from the outer peripheral surface, so that theincrease in the surface temperature of the sliding surface can befurther suppressed.

Advantageous Effects of Invention

According to the present invention, the cooling performance for thesliding surfaces can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a swash plate typeliquid-pressure apparatus according to an embodiment of the presentinvention,

FIG. 2 is a front view showing a cylinder block included in the swashplate type liquid-pressure apparatus shown in FIG. 1 when viewed fromfront.

FIG. 3 is a cross-sectional view taken along line A-A of FIG. 2 andshowing the cylinder block.

FIG. 4 is one example of a diagram of an oil-pressure circuit around theswash plate type liquid-pressure apparatus.

FIG. 5A is a diagram showing a piston located at a bottom dead center.FIG. 5B is a graph showing surface temperatures at respective positionsof a sliding surface of the cylinder block in a state shown in FIG. 5A.FIG. 5C is a graph showing oil pressures of the sliding surface of thecylinder block in the state shown in FIG. 5A.

FIG. 6 is a front view showing a cooling structure of the cylinder blockof another embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a swash plate type liquid-pressure apparatus 1 according toan embodiment of the present invention will be explained in reference tothe above drawings. The swash plate type liquid-pressure apparatus 1explained below is just one embodiment of the present invention. Thepresent invention is not limited to the embodiment, and additions,eliminations and modifications may be made within the spirit of thepresent invention.

Swash Plate Type Liquid-Pressure Apparatus

In construction machinery, industrial machinery, and ships, the swashplate type liquid-pressure apparatus 1 is provided to drive respectivedevices and actuators included therein. Examples of the constructionmachinery are oil-pressure shovels, cranes, and bulldozers. Examples ofthe industrial machinery are land devices, such as oil-pressure units,pressing machines, ironmaking machines, and injection molding machines.The swash plate type liquid-pressure apparatus 1 is a so-called swashplate type motor/pump and has the function of a liquid-pressure motorconfigured to cause a rotated object included in the industrialmachinery or ship to rotate or the function of a liquid-pressure pumpconfigured to supply a pressure liquid to an actuator included in theindustrial machinery or ship to activate the actuator. In the followingexplanation, for convenience of explanation, a fluid used is operatingoil, and the swash plate type liquid-pressure apparatus 1 will beexplained as an oil-pressure motor.

As shown in FIG. 1, an oil-pressure motor 1 that is the swash plate typeliquid-pressure apparatus 1 is a high-speed rotation oil-pressure motorincluding a rotating shaft 11 and capable of rotating the rotating shaft11 at high rotation speed. In addition to the rotating shaft 11, theoil-pressure motor 1 further includes a cylinder block 12, a pluralityof pistons 13, a plurality of shoes 14, a swash plate 15, and a valveplate 16, and these components are accommodated in a casing 17. Therotating shaft 11 extends in a front-rear direction so as to penetratethe casing 17 and is rotatably supported by bearings 18 and 19 at frontand rear end portions of the casing 17. The cylinder block 12 is fittedon the rotating shaft 11 so as to be located on a rear end portion of amiddle portion of the rotating shaft 11.

The cylinder block 12 is formed in a substantially cylindrical shape,and an axis thereof is located so as to coincide with an axis L1 of therotating shaft 11. The cylinder block 12 is integrally splined to therotating shaft 11 and is not relatively rotatable with respect to therotating shaft 11. A front end portion of an outer peripheral surface 12a of the cylinder block 12 is reduced in thickness toward a radiallyinner side over the entire periphery in the circumferential direction,and a cooling structure 30 is formed on the front end portion of theouter peripheral surface 12 a. Details of the configuration of thecooling structure 30 will be described below. A plurality of cylinders20 are formed on the cylinder block 12. As shown in FIG. 2, thecylinders 20 are arranged at regular intervals in the circumferentialdirection. As shown in FIG. 3, the cylinders 20 extend in parallel withthe axis L1. Each of the cylinders 20 is a hole defined by a slidingsurface having a circular cross section and a bottom surface and has anopening on a front end surface (piston insertion end surface) of thecylinder block 12. The pistons 13 are respectively inserted through theopenings to fit in the cylinders 20.

Each of the pistons 13 is formed in a substantially columnar shape andperforms the reciprocating sliding in the front-rear direction whilesliding on the sliding surface 12 b defining the cylinder 20. Acylindrical sleeve (not shown), such as a copper bushing, may fit in thecylinder 20. In this case, the piston 13 slides on an inner peripheralsurface of the sleeve, and the sliding surface on which the piston 13slides denotes the inner peripheral surface of the sleeve. In thefollowing explanation, the sleeve does not fit in the cylinder 20.However, the same is true for a case where the sleeve fits in thecylinder 20.

An outer diameter of the piston 13 is slightly smaller than an innerdiameter of the cylinder 20, and a clearance is formed around the piston13, that is, the clearance is formed between the piston 13 and thesliding surface 12 b. Further, the piston 13 includes a sphericalsupport portion 13 a at its front end portion. The spherical supportportion 13 a projects from the cylinder 20 regardless of the position ofthe piston 13. An outer surface of the spherical support portion 13 a isformed in a substantially spherical shape, and the shoe 14 is attachedto the spherical support portion 13 a.

Each of the shoes 14 is formed in a substantially bottomed cylindricalshape, and an inner surface thereof is formed in a partial sphericalshape corresponding to the spherical support portion 13 a. The sphericalsupport portion 13 a of the piston 13 fits in the shoe 14, and thepiston 13 is rotatable around a center point that is the center of thespherical support portion 13 a. The shoe 14 includes, at its bottomportion, a flange 14 a projecting in a radially outward direction and isarranged on the swash plate 15 such that the bottom portion thereofcontacts the swash plate 15.

The swash plate 15 is formed in a substantially circular plate shape.The swash plate 15 is provided in the casing 17 so as to be inclinedsuch that an upper portion thereof is located on a rear side, and therotating shaft 11 penetrates the vicinity of the center of the swashplate 15. The swash plate 15 is provided on a front side of the cylinderblock 12 and includes a supporting plate 21 on the cylinder block 12side. The supporting plate 21 is formed in an annular shape, and theplurality of shoes 14 are arranged on the supporting plate 21 at regularintervals in the circumferential direction. A retainer plate 22 isprovided on the plurality of shoes 14 so as to press the shoes 14against the supporting plate 21.

The retainer plate 22 is formed in a substantially annular shape, andthe rotating shaft 11 penetrates the center of the retainer plate 22 soas to be relatively rotatable with respect to the retainer plate 22.Attachment holes 22 a, the number of which is equal to the number ofshoes 14, are formed on the retainer plate 22. The attachment holes 22 aare arranged at regular intervals in the circumferential direction.Opening portions of the shoes 14 are respectively inserted in theattachment holes 22 a, and the retainer plate 22 contacts the flanges 14a. Thus, the retainer plate 22 and the supporting plate 21 sandwich theflanges 14 a. A spherical bushing 23 is inserted in an inner hole of theretainer plate 22. The spherical bushing 23 is formed in a substantiallycylindrical shape and is externally attached to the rotating shaft 11and the cylinder block 12. The spherical bushing 23 is biased toward thesupporting plate 21 by a plurality of pressing springs 40 provided atthe cylinder block 12, and the retainer plate 22 is pressed against thesupporting plate 21 by the spherical bushing 23.

An upper portion of the swash plate 15 on which the plurality of shoes14 are arranged as above is coupled to a regulator 24 provided at anupper portion of the casing 17. The regulator 24 includes a plunger 25configured to be movable in the front-rear direction, and the swashplate 15 is coupled to the plunger 25. Therefore, by causing the plunger25 to move in the front-rear direction, the inclination angle of theswash plate is changed. Thus, the strokes of the pistons 13 can beadjusted, and the volumes of oil chambers 20 a of the cylinders 20 canbe changed. Each of the oil chambers 20 a is a space in the cylinder 20,the space being located on the rear side of a rear end surface of thepiston 13.

Cylinder ports 26 respectively connected to the oil chambers 20 a areformed at the cylinder block 12. One cylinder port 26 is formed for onecylinder 20, that is, the cylinder ports 26 correspond one-to-one to thecylinders 20. The cylinder ports 26 open on a rear end surface of thecylinder block 12, and the valve plate 16 is provided on this rear endsurface.

The valve plate 16 is an annular plate-shaped member and is locatedbetween the cylinder block 12 and a rear end portion of the casing 17.The valve plate 16 is fixed to the casing 17 by pin members, not shown,so as not to be relatively rotatable with respect to the casing 17. Therotating shaft 11 is inserted through an inner hole of the valve plate16, and the rotating shaft 11 and the valve plate 16 are configured tobe relatively rotatable with respect to each other. An inlet port 16 aand an outlet port 16 b are formed on the valve plate 16 located asabove.

Each of the inlet port 16 a and the outlet port 16 b is formed in asubstantially circular-arc shape. The inlet port 16 a and the outletport 16 b are located so as to be spaced apart from each other in thecircumferential direction. Each of the inlet port 16 a and the outletport 16 b penetrates the valve plate 16 in the thickness direction.Regarding each of the inlet port 16 a and the outlet port 16 b, itsopening located on the cylinder block 12 side is connected to some ofthe cylinder ports 26. By rotating the cylinder block 12, a destinationto which the cylinder port 26 is connected is alternately switchedbetween the inlet port 16 a and the outlet port 16 b. The other openingof the inlet port 16 a is connected to a high-pressure passage 27 shownin FIG. 4, and the other opening of the outlet port 16 b is connected toa low-pressure passage 28 shown in FIG. 4. To be specific, by rotatingthe cylinder block 12, each of the cylinders 20 is alternately connectedto the high-pressure passage 27 and the low-pressure passage 28. In FIG.1, for convenience of explanation, the positions of the inlet port 16 aand the outlet port 16 b are shifted in the circumferential directionfrom the actual positions of the inlet port 16 a and the outlet port 16b. A circuit configuration shown in FIG. 4 is one example for furtherimproving the cooling effect. Even without this configuration, thecooling effect can be obtained by oil in a case.

A communication passage 29 shown in FIG. 4 is formed at the casing 17.An internal space of the casing 17 and the low-pressure passage 28 areconnected to each other through the communication passage 29. With this,a certain amount of operating oil flowing through the low-pressurepassage 28 can be introduced through the communication passage 29 to theinternal space of the casing 17 to be utilized as a cooling liquid.Thus, the rotating shaft 11, the cylinder block 12, the pistons 13, andthe like can be cooled by low-pressure, low-temperature operating oil.

In the oil-pressure motor 1 configured as above, while the piston 13moves from a top dead center where the piston retracts most in thecylinder 20 to a bottom dead center where the piston 13 projects mostfrom the cylinder 20, the operating oil flowing through thehigh-pressure passage 27 is suctioned to the oil chamber 20 a throughthe inlet port 16 a. With this, the piston 13 is pressed forward by theoperating oil. As a result, the shoe 14 is pressed against the swashplate 15. Since the swash plate 15 is inclined, the pressed shoe 14slides on the swash plate 15 so as to move downward and revolves aroundthe axis L1 in one direction along the circumferential direction. Withthis, the rotational force around the axis L1 is applied to the cylinderblock 12, and the cylinder block 12 and the rotating shaft 11 rotatearound the axis L1.

When the piston 13 is located between the bottom dead center and the topdead center, the oil chamber 20 a is connected to the low-pressurepassage 28 via the outlet port 16 b. By the rotation of the cylinderblock 12, the shoe 14 slides on the swash plate 15 so as to move upwardand revolves around the axis L1 in one direction along thecircumferential direction. When the shoe 14 moves upward, the piston 13is pressed backward. With this, the operating oil in the oil chamber 20a is discharged through the outlet port 16 b to the low-pressure passage28. As above, in the oil-pressure motor 1, by suctioning and ejectingthe operating oil, the pistons 13 perform the reciprocating sliding inthe front-rear direction, and the cylinder block 12 and the rotatingshaft 11 are rotated around the axis L1.

As described above, in the oil-pressure motor 1 configured to repeat thesuction and discharge of the operating oil, when suctioning anddischarging the operating oil, the pistons 13 slide on the slidingsurfaces 12 b to perform the reciprocating sliding in the front-reardirection. Therefore, frictional heat is generated on the slidingsurfaces 12 b when sliding, and the surface temperature of the slidingsurface 12 b, especially the surface temperature of a region on theopening side, increases. The clearance is formed between the outersurface of the piston 13 and the sliding surface 12 b. By utilizing theoperating oil leaking from the clearance as lubricating oil, the pistons13 are lubricated. Thus, the frictional heat generated on the slidingsurfaces 12 b is reduced, and the sliding surfaces 12 b are cooled bythe lubricating oil. As above, in the oil-pressure motor 1, by providingthe clearances, the increase in the surface temperatures of the slidingsurfaces 12 b is suppressed. However, to further suppress the increasein the surface temperatures, the oil-pressure motor 1 further includesthe cooling structure 30 of the cylinder block 12.

Cooling Structure of Cylinder Block

The cooling structure 30 of the cylinder block 12 includes coolingdepressions 31. As shown in FIG. 2, the cooling depressions 31 arerespectively formed at dividing walls 32 each located between twoadjacent cylinders 20 and extend from the front end surface of thecylinder block 12 toward the rear end surface thereof in parallel withthe axis L1. In the present embodiment, a tip end of the coolingdepression 31 is located on the cylinder block 12 front end surface sideof the vicinity of the rear end surface of the piston 13 located at thebottom dead center, that is, the tip end of the cooling depression 31 islocated on the front side of the vicinity of the rear end surface of thepiston 13 located at the bottom dead center (see FIG. 3). Each of thedividing walls 32 denotes an entire wall (region shown by a diamond netpattern in FIG. 2) located between straight lines L2 and L3 respectivelyextending from the center of the cylinder block 12 through one of thecenters of two adjacent cylinders 20 to the outer peripheral surface 12a and from the center of the cylinder block 12 through the other centerto the outer peripheral surface 12 a.

FIGS. 5B and 5C are graphs respectively showing surface temperatures andoil pressures at respective positions on the sliding surface 12 b whenthe piston 13 is located at the bottom dead center (see FIG. 5A). InFIG. 5B, a vertical axis shows a surface temperature T of the slidingsurface 12 b, and a horizontal axis shows a distance d from the frontend surface of the cylinder block 12. In FIG. 5C, a vertical axis showsan oil pressure P applied to the sliding surface 12 b, and a horizontalaxis shows the distance d from the front end surface of the cylinderblock 12. As is clear from FIG. 5B, regarding the surface temperature ofthe sliding surface 12 b, a portion located on the rear side of the rearend surface of the piston 13 (to be specific, a portion between adistance d1 and a distance d2) is cooled by the operating oil in the oilchamber 20 a, so that the potion is maintained at a substantiallyconstant temperature. In contrast, regarding a portion located on thecylinder 20 opening side of the rear end surface of the piston 13 (to bespecific, a portion between a distance 0 and the distance d1), thecooling effect by the operating oil in the clearance is small.Therefore, the surface temperature increases toward the opening side,and the surface temperature is the highest in the vicinity of theopening, that is, highest on the front end surface of the cylinder block12.

In addition, as is clear from FIG. 5C, regarding the oil pressureapplied to the sliding surface 12 b, since a region of the slidingsurface 12 b, the region being located on the rear side of the rear endsurface of the piston 13, forms the oil chamber 20 a, the oil pressureapplied to this region is substantially equal to the pressure of theoperating oil suctioned through the inlet port 16 a. In contrast,regarding the oil pressure applied to a region of the sliding surface 12b, the region being located on the front side of the rear end surface ofthe piston 13, since a front portion of the clearance is connected tothe casing 17, the oil pressure decreases toward the opening side. Theoil pressure on the opening side is decreased up to the pressure in thecasing, that is, a drain pressure.

As above, the surface temperature of the sliding surface 12 b and theoil pressure applied to the sliding surface 12 b are different betweenthe front side and rear side of the rear end surface of the piston 13located at the bottom dead center. When the piston 13 is located at thebottom dead center, a high pressure is applied to the sliding surface 12b over a widest range. As in the present embodiment, by arranging thetip end of the cooling depression 31 on the cylinder block 12 front endsurface side of the rear end surface of the piston 13, the coolingperformance of a region where the surface temperature becomes high canbe improved while increasing the stiffness of a region where the oilpressure applied to the sliding surface 12 b becomes high. With this,damages by the burnout of the cylinder 12 and the piston 13 can beprevented without decreasing a service limit pressure of the operatingoil.

When viewed from front as in FIG. 2, the cooling depression 31 extendingas above is bent so as to project toward a radially inner side. A regionof the dividing wall 32 is reduced in thickness, the region beinglocated between the sliding surface 12 h and the outer peripheralsurface 12 a. As above, by reducing the thickness of the dividing wall32, that is, by reducing a thickness t of a portion between the slidingsurface 12 b and the outer peripheral surface 12 a, the heat generatedon the sliding surface 12 b can be quickly transferred to the outerperipheral surface 12 a subjected to the low-temperature operating oil.Thus, the heat can be released to the low-temperature operating oil, andthe temperature increase of the sliding surface 12 b can be suppressed.Therefore, even if the cylinder block 12 is increased in speed, theoperating oil (lubricating oil) in the clearance between the slidingsurface 12 b and the piston 13 can be prevented from increasing intemperature and exceeding the transition temperature, and the burnout ofthe sliding surface 12 b by the decrease in the lubrication performanceof the operating oil can also be prevented. In addition, the surfacetemperature of the sliding surface 12 b can be reduced withoutincreasing the clearance between the sliding surface 12 b and the piston13 and forming oil grooves on the sliding surface 12 b. Therefore, thecooling performance improves without decreasing the performance of themotor.

Hereinafter, the shape of the cooling depression 31 will be furtherexplained. The cooling depression 31 is formed to satisfy 0.02D≦tmin≦0.3 D, where tmin denotes a minimum thickness of the portionbetween the sliding surface 12 b and the outer peripheral surface 12 b,and D denotes the inner diameter of the cylinder 20. More specifically,the cooling depression 31 is formed to satisfy 0.02 D≦t≦0.3 D, where tdenotes a thickness of a portion between the outer peripheral surface 12a and a region 12 c (corresponding to a region located on the outerperipheral surface 12 a side in the sliding surface 12 b) located on aradially outer side in the sliding surface 12 b, and D denotes the innerdiameter of the cylinder 20. The region 12 c located on the radiallyouter side is a region spreading from an intersection point A1 towardboth directions along the circumferential direction, the intersectionpoint A1 being one of two points where the straight line L2 and thesliding surface 12 b intersect with each other and being located on theradially outer side. When the cylinder block 12 is rotated at highspeed, the piston 13 pressed by the centrifugal force contacts theregion 12 c. Thus, the piston 13 slides on the sliding surface 12 b in astate where the piston 13 contacts the region 12 c by the largecentrifugal force. Therefore, high frictional heat is generated on theregion 12 c. For example, the region 12 c is a region spreading from theintersection point A1 as a center toward both directions along thecircumferential direction and having a center angle α, and the centerangle α satisfies 30°≦α≦180°. By external factors, such as the rotatingspeed and rotational direction of the cylinder block 12, and vibrations,the position where the piston 13 contacts may be out of the rangedefined by the center angle α. Therefore, the cooling depression 31 maybe formed such that the thickness t around the sliding surface 12 bsatisfies 0.02 D≦t≦0.3 D over a wider range than the range defined bythe center angle α (for example, see FIG. 6 described below).

By setting the thickness t to 0.3 D or less, the cooling performance ofthe sliding surface 12 b, especially the region located on the openingside, can be improved, and the increase in the surface temperature ofthe sliding surface 12 b can be suppressed. With this, the temperatureincrease of the operating oil (lubricating oil) flowing through theclearance between the sliding surface 12 b and the piston 13 can besuppressed, and the temperature of the operating oil can be preventedfrom increasing and exceeding the transition temperature. Therefore, theburnout of the sliding surface 12 b by the decrease in the lubricationperformance of the operating oil can be prevented.

Since the surface temperature of the sliding surface 12 b can bedecreased without increasing the clearance between the sliding surface12 b and the piston 13 and forming oil grooves on the sliding surface 12b, the cooling performance improves without decreasing the performanceof the motor. Further, by setting the thickness t to 0.02 D or more, thestiffness in the vicinity of the opening side of the region 12 c locatedon the radially outer side in the sliding surface 12 b can be secured.Even when the piston 13 performs the reciprocating sliding at high speedin operation, the damages in the vicinity of the opening side can beprevented.

Other Embodiments

In the above embodiment, a bottom surface of the cooling depression 31is bent in an arch shape. However, the bottom surface does not have tohave the arch shape. For example, as shown in FIG. 6, a coolingdepression 31A of a cooling structure 30A may be formed along thesliding surface 12 b to have a sharp tip shape, so that the thickness tof an entire semicircle located on the radially outer side in thesliding surface 12 b may become uniform. In addition, the shape of thecooling depression 31 does not have to be bent. The bottom surface maybe flat, or the bottom surface may be formed like a fin by formingdepressions and projections thereon. Further, the vicinity of the tipend of the bottom surface of the cooling depression 31 is bent towardthe tip end so as to be located on the radially outer side. However, thevicinity of the tip end of the bottom surface does not have to be bentand may be flat up to the tip end (for example, see reference sign 41shown by a chain double-dashed line in FIG. 3). Further, in the aboveembodiment, as shown in FIG. 3, the tip end of the cooling depression 31is located between the front end surface of the cylinder block 12 andthe rear end surface of the piston 13 located at the bottom dead center.However, the tip end of the cooling depression 31 may be located in thevicinity of the rear end surface.

The above embodiment has explained a case where the swash plate typeliquid-pressure apparatus 1 is the oil-pressure motor. However, asdescribed above, the swash plate type liquid-pressure apparatus 1 may bethe oil-pressure pump. In this case, the cylinder block 12 is rotated byrotating the rotating shaft 11 by an electric motor or an engine. By therotation of the cylinder block 12, the pistons 13 perform thereciprocating sliding. In the oil-pressure pump, the outlet port 16 b isconnected to the high-pressure passage 27, and the inlet port 16 a isconnected to the low-pressure passage 28. While the piston 13 moves fromthe top dead center to the bottom dead center, the operating oil issuctioned through the inlet port 16 a to the oil chamber 20 a. While thepiston 13 moves from the bottom dead center to the top dead center, thesuctioned operating oil is compressed and ejected through the outletport 16 b to the high-pressure passage 27.

As above, even in the case of the oil-pressure pump, the pistons performthe reciprocating sliding in the cylinders 20 and slide on the slidingsurfaces 12 b. Therefore, as with the oil-pressure motor 1, the heat isgenerated on the sliding surfaces 12 b. On this account, even in thecase of using the swash plate type liquid-pressure apparatus 1 as theoil-pressure pump, the same operational advantages as the oil-pressuremotor 1 can be obtained by the cooling structure 30 of the cylinderblock 12.

Further, the above embodiment has explained the swash plate typeliquid-pressure apparatus 1. However, the cooling structure 30 of thecylinder block 12 may be applied to an inclined shaft typeliquid-pressure apparatus. However, in the inclined shaft typeliquid-pressure apparatus, even if the speed is increased, the surfacetemperature of the sliding surface 12 b does not increase, unlike theswash plate type liquid-pressure apparatus 1. Therefore, higheroperational advantages are obtained in the case of the swash plate typeliquid-pressure apparatus. Although the oil is used as the operatingliquid, other liquids, such as water, may be used as the operatingliquid.

REFERENCE SIGNS LIST

1 oil-pressure motor

12 cylinder block

12 a outer peripheral surface

12 b sliding surface

12 c region

13 piston

17 casing

20 cylinder

27 high-pressure passage

28 low-pressure passage

29 communication passage

30 cooling structure

31 cooling depression

32 dividing wall

1. A cooling structure of a cylinder block configured such that: aplurality of cylinders each including an opening on a piston insertionend surface of the cylinder block are formed on the cylinder block; andwhen the cylinder block is rotated, pistons respectively inserted in thecylinders perform reciprocating sliding, the cooling structurecomprising a plurality of cooling depressions formed on an outerperipheral surface of the cylinder block, wherein each of the coolingdepressions extends from the piston insertion end surface on a dividingwall located between the two adjacent cylinders and is formed byreducing a thickness of the dividing wall so as to reduce a thickness ofa portion between the outer peripheral surface of the cylinder block anda sliding surface on which the piston slides.
 2. The cooling structureaccording to claim 1, wherein: each of the pistons performs thereciprocating sliding between a top dead center and a bottom dead centerin the cylinder; and each of the cooling depressions is formed so as toextend from the piston insertion end surface in parallel with thecylinder and is formed such that a tip end of the cooling depression islocated on the piston insertion end surface side of a vicinity of an endsurface of the piston located at the bottom dead center, the end surfacebeing located in the cylinder.
 3. The cooling structure according toclaim 1, wherein each of the cooling depressions is formed so as tosatisfy 0.02 D≦tmin≦0.3 D, where train denotes a minimum thickness ofthe portion between the outer peripheral surface of the cylinder blockand the sliding surface, and D denotes an inner diameter of thecylinder.
 4. A swash plate type liquid-pressure apparatus configured tobe connected to a low-pressure passage through which a low-pressureoperating liquid flows and a high-pressure passage through which ahigh-pressure operating oil flows and further configured such that: thecylinder block is rotated by supplying the operating liquid through thehigh-pressure passage to the cylinders and discharging the operatingliquid from the cylinders to the low-pressure passage; or by rotatingthe cylinder block, the operating liquid is suctioned through thelow-pressure passage to the cylinders, and the operating liquid is thencompressed and ejected to the high-pressure passage, the swash platetype liquid-pressure apparatus comprising the cooling structureaccording to claim
 1. 5. The swash plate type liquid-pressure apparatusaccording to claim 4, further comprising a casing configured toaccommodate the cylinder block, wherein the casing is connected to thelow-pressure passage through a communication passage, and low-pressureoperating oil in the low-pressure passage is introduced to the casing.6. A swash plate type liquid-pressure apparatus configured to beconnected to a low-pressure passage through which a low-pressureoperating liquid flows and a high-pressure passage through which ahigh-pressure operating oil flows and further configured such that: thecylinder block is rotated by supplying the operating liquid through thehigh-pressure passage to the cylinders and discharging the operatingliquid from the cylinders to the low-pressure passage; or by rotatingthe cylinder block, the operating liquid is suctioned through thelow-pressure passage to the cylinders, and the operating liquid is thencompressed and ejected to the high-pressure passage, the swash platetype liquid-pressure apparatus comprising the cooling structureaccording to claim
 2. 7. A swash plate type liquid-pressure apparatusconfigured to be connected to a low-pressure passage through which alow-pressure operating liquid flows and a high-pressure passage throughwhich a high-pressure operating oil flows and further configured suchthat: the cylinder block is rotated by supplying the operating liquidthrough the high-pressure passage to the cylinders and discharging theoperating liquid from the cylinders to the low-pressure passage; or byrotating the cylinder block, the operating liquid is suctioned throughthe low-pressure passage to the cylinders, and the operating liquid isthen compressed and ejected to the high-pressure passage, the swashplate type liquid-pressure apparatus comprising the cooling structureaccording to claim 3.